In general, the complete and efficient removal of a liquid from a surface of a substrate is a multiply repeated step in e.g. the fabrication process of integrated circuits. Such a step can be performed after a wet etching step or a wet cleaning step or a wet rinsing step or any other step used in said fabrication process wherein said substrate is treated or exposed or immersed in a liquid. Said substrate can be a semiconductor wafer or a part thereof or a glass slice or any other slice of an insulating or conductive material.
The manufacturing of integrated circuits evolves towards processing of each substrate individually rather than in batches of several substrates. In the state of the art of IC manufacturing, most processing steps, such as implantation steps, deposition steps, are already performed in a single substrate mode. On the other hand, wet processing steps, such as cleaning steps and subsequent liquid removal steps, are typically performed in a batch mode because of a lack of appropriate alternatives. Therefore, differences in waiting times are created for each individual substrate between a wet processing step, performed in a batch mode and another processing step, performed in a single substrate mode. Such variability is undesirable with regard to process control. Moreover this mixed batch and single substrate processing increases the cycle time, which is also undesirable. Therefore, there is a general interest in the development of competitive single substrate wet processing steps. Particularly, one of the major challenges regarding single wafer wet processing is a method for removing a liquid from both sides of a substrate. There are two major requirements to be fulfilled for such a method. First, it should work sufficiently fast. In state of the art production lines, a substrate is processed typically every 2 to 3 minutes. Ideally, in order to avoid equipment duplication, the liquid removal should be completed in about such a time frame. Another requirement is related to the preferred orientation of the substrate. State of the art processing equipment and transportation tools are developed to handle substrates in a horizontal position. Therefore in order to avoid additional substrate handling, it would be desirable to perform the wet processing steps using horizontally positioned substrates.
In the European Patent EP 0 385 536 B1, a method is disclosed of drying substrates after treatment in a liquid by pulling said substrate slowly out of said liquid. However, to be effective, this known method, which is based on the marangoni principle, requires that the substrates are pulled out of the liquid in an upright position, i.e. a surface of said substrate is about perpendicular to the surface of the liquid bath (as can be seen in FIGS. 1 to 6 of the European Patent EP 0 385 536 B1). This handling is incompatible with the majority of the other process steps where the equipment and transportation tools are developed to handle horizontal positioned substrates.
In the U.S. Pat. No. 5,271,774, a spin-drying technique is disclosed which is able to handle horizontal positioned substrates. In fact, several small liquid islands are formed being removed from the substrate by a rotary movement. It is known that such a spin-drying technique leaves undesirable residues, often referred to as drying marks, on the substrate surface, particularly on surfaces having mixed hydrophilic and hydrophobic regions. Furthermore, this technique is not suited to remove residual contamination in the form of particles.
It is clear from the above discussion that there is a general interest in single wafer wet processing and drying. Therefore, an efficient drying technique which also has good cleaning capabilities is a prerequisite. For some inherently dirty processes such as chemical mechanical polishing, cleaning steps making use of a mechanical force are used nowadays. For instance, CMP machines typically used for planarizing or smoothing a semiconductor wafer surface employ a polish slurry containing generally sub-micron sized abrasive particles of a material such as silica or alumina. Large quantities of such particles, along with particles removed from the wafer itself during the polishing operation, may be found on a polished wafer surface. These particles must be removed in a cleaning step or they may drastically affect device yield.
For instance, in the U.S. Pat. No. 5,581,837, disk-shaped objects, e.g. wafers, are transported one at the time between a set of rotating brushes so as to be cleaned thereby. In fact, in the presence of a cleaning liquid, a mechanical force is exerted on the surface of the wafer by means of the rotating brushes to get the wafers cleaned. This cleaning step can be followed subsequently by a rinsing step and a separate drying step. In this drying step, IR lamps are used to performing the drying. It is known that such a technique leaves drying marks on the surface of the substrate.